1. Field of the Invention
The present invention relates to a motor speed controlling method, and more particularly, to a method for controlling speed of a motor for driving an industrial robot.
2. Description of the Related Art
In general, an industrial robot includes mechanical parts for moving a target object on a three-dimensional space or performing a requested job. The robot includes a body which is a central portion for supporting a robot, a manipulator having an arm for moving an end effector into which a tool necessary for a requested job is fitted, to a particular position in a working area, a controller for controlling the manipulator, and a power source for supplying an electric power to the body, the manipulator and the controller. A servo motor is mounted in the arm of the manipulator and the controller controls the servo motor to perform an allocated job.
Generally, a controller includes a main controller, a position controller and a servo driver for driving a servo motor. The main controller produces a speed command profile based on a current position and a user operation command such as a target position input by a user and transfers the produced speed command profile to the position controller. The position controller controls the servo driver based on the received speed command profile and drives the servo motor to move an arm from a certain point to another point. Meanwhile, the position controller receives a current position from an encoder mounted in the servo motor and feedback-controls the position of the arm.
FIGS. 1a and 1a' show user command paths according to the conventional art. FIGS. 1b and 1b' show speed command profiles with respect to FIGS. 1a and 1a', respectively according to the conventional art. FIGS. 1c and 1c' show acceleration paths with respect to FIGS. 1a and 1a', respectively, according to the conventional art.
In a conventional robot, the main controller produces a path plan such as a speed trace in which a final target value is planned according to a movement command from a user as shown in FIGS. 1a and 1a'. Then, the main controller produces a trajectory plan profile such as a speed command profile in which an actual acceleration and deceleration is performed based on the path plan as shown in FIGS. 1b and 1b'.
In other words, according to the conventional robot, the main controller produces a path plan to control a servo motor which drives a manipulator, and then produces a trajectory plan profile based on the path plan. As a result, a response to a user command is not fast. Further, even though a trajectory plan profile is given as a continuous value of a smooth curve, a number of points where a differentiation is impossible exist in a jerk such as an acceleration profile as shown in FIGS. 1c and 1c'. Since an acceleration change rate is sharp in the vicinity of the differentiation-impossible points, vibration and noise are generated in the motor. The vibration and noise lowers reliability with respect to performance of the robot and gives a shock to mechanical components including a motor, thereby shortening the lifetime of the robot. The vibration and noise is conspicuously severe in a large-scale mechanical apparatus such as an industrial robot which is greatly influenced by the inertia and gravity.